GABAergic inhibition is essential for circuit development and function. In the developing visual system, GABAergic inhibition determines the onset and close of the critical period for ocular dominance plasticity. Furthermore, the development of GABAergic synaptic transmission is regulated by visual experience. These studies suggest that sensory input activity regulates the development of GABAergic neurons and their synaptic connections, and that, in turn, inhibitory GABAergic synaptic transmission regulates the development and plasticity of visual circuits. Despite this postulated pivotal role for inhibition in circuit development, the inability to visualize and perturb connectivity of GABAergic inhibitory neurons in an intact developing circuit has prevented a critical evaluation of this hypothesis. We have developed tools to address both of these limitations and now propose experiments to determine mechanisms controlling the development of GABAergic neurons and their synaptic connections in the intact Xenopus tadpole optic tectum and to determine how GABAergic inputs control the development of visual receptive fields and visually guided behavior. To visualize GABAergic neurons in the intact animal, we will use the VGAT promoter to drive expression of fluorescent proteins (FPs) and other genes of interest specifically in GABAergic neurons. We will decrease synaptic transmission onto GABAergic neurons by VGAT-driven expression of peptides which we have shown specifically inhibit GABAergic or glutamatergic synaptic transmission in a cell autonomous manner. In Aim 1, we will collect time-lapse images of individual GABAergic tectal neurons expressing FP and FP-tagged synaptic proteins to identify mechanisms that control GABAergic neuronal development. We will test the role of glutamatergic and GABAergic synaptic transmission in GABAergic neuron development by expressing peptides to block synaptic transmission onto the imaged cell. In Aim 2, we will combine in vivo time-lapse imaging, to identify stable and dynamic dendrites, and retrospective serial section electron microscopy (EM) to determine the synaptic rearrangements that occur during GABAergic neuronal development. Data from Aims 1 and 2 will demonstrate the activity-dependent mechanisms that control the morphological development and synaptic connectivity of GABAergic neurons in vivo. In Aims 3 and 4 we will test the effect of blocking inhibitory and excitatory transmission on tectal visual receptive field properties, using cell-attached and whole recordings, and a visual avoidance assay, that will extend our studies into the behavioral arena. The aberrant development and function of inhibitory circuits is thought to underlie a variety of developmental neurological disorders, including autism, schizophrenia and depression, however the basic mechanisms governing the development of inhibitory circuits are relatively unknown. The experiments proposed here should illuminate activity-dependent mechanisms of GABAergic circuit development in vivo.